Postfix stable release 2.5.6, 2.4.10 and 2.3.16 available

Postfix stable releases 2.5.6, 2.4.10 and 2.3.16 catch up on fixes
that were applied in the past three months. Postfix versions 2.2
and earlier are no longer updated.

- Postfix 2.5: the SMTP server did not ask for a client certificate
with "smtpd_tls_req_ccert = yes". Reported by Rob Foehl.

- Postfix 2.5, 2.4 and 2.3: avoid reduced TCP performance when
reusing an SMTP connection with a larger than 4096-byte TCP MSS
value. In practice, this could happen only with loopback (localhost)
connections.